“Osteoporosis is a systemic skeletal disease characterized by low bone mass and macroarchitectural deterioration of bone tissue, which leads to bone fragility and susceptibility of fracture” (Blake, et al., “The evaluation of osteoporosis: dual energy x-ray absorptiometry and ultrasound in clinical practice.” Martin Dunitz Ltd 1999.). According to the National Osteoporosis Foundation one in two women and one in eight men over the age of fifty will have an osteoporotic-related fracture in their lifetime. Eight million American women and two million men have osteoporosis and millions more have low bone density. The use of noninvasive bone densitometry has resulted in significant improvement in the early detection of osteoporosis.
Dual x-ray absorptiometry (DXA) is the most often used method in the measurement of bone mineral density because of the low cost, low radiation dose and the possibility of measurement at the anatomical site where the fractures most commonly occur (femur, spine). (See Genant, et al., “Noninvasive assessment of bone mineral and structure: state of the art.” J Bone Miner Res. 11: 707-30 (1996)). By using only two energies, the dual x-ray absorptiometry method is limited to distinguishing tissue composition to two components (muscle and bone or muscle and adipose tissue). To correct for different tissue composition (varying proportions of lean and adipose tissue), the fat fraction is calculated over the soft tissue part of the patient where no bone is present. The assumption is made that adipose tissue is uniformly distributed and the soft tissue composition adjacent to the bone is the same as the soft tissue overlying the bone. However, in fact, fat is not uniformly distributed and therefore, this assumption can introduce errors of greater than 10% in a significant percentage of the patient population. (See Svendsen, et al. “Impact of soft tissue on in-vivo accuracy of bone mineral measurements in the spine, hip and forearm: a human cadaver study.” J Bone Miner Res 10: 868-73, (1995); Tothill, et al. “Errors due to non-uniform distribution of fat in dual x-ray absorptiometry of the lumbar spine.” Br J Radiol 65:807-13, (1992); Farrell, et al. “The error due to fat inhomogenity in lumbar spine bone mineral measurements.” Clin. Phys. Physiol. Meas. 10 57-64, (1989)).
It would be desirable to provide a method and apparatus that can, from the energy distribution of the x-ray spectra, incident and transmitted through the patient, determine the areal densities of the three tissue components: bone, fat and soft tissue. The present invention addresses this need and others in the following manner.